1. Field of the Invention
The present invention relates to a method for the contamination-free size reduction of semiconductor material, especially silicon.
2. The Prior Art
The production of solar cells or electronic components such as storage elements or microprocessors requires hyperpure semiconductor material. The specifically introduced dopants are the only impurities which a material of this type should contain. It is therefore desirable to keep. the concentrations of harmful impurities as low as possible. It is often observed that semiconductor material, which had earlier been prepared in a hyperpure state, is later contaminated in the course of further processing to produce the end products. Thus, laborious purification steps become necessary in order to recover the original purity. Atoms of foreign metals, which are incorporated into the crystal lattice of the semiconductor material, interfere with the charge distribution. These foreign metals may reduce the function of the subsequent component or lead to the failure thereof. Consequently, contaminations of the semiconductor material by metallic impurities are particularly to be avoided. This applies especially to silicon, which in the electronics industry is the most commonly used semiconductor material.
Hyperpure silicon is obtained, for example, by thermal decomposition of silicon compounds which are highly volatile, such as trichlorosilane, and are therefore easily purified by distillation processes. In this process, the hyperpure silicon is obtained as polycrystalline rods having typical diameters of 70 to 300 mm and lengths of 500 to 2500 mm. A large proportion of the rods is used for the production of crucible-drawn single crystals or for producing strips and sheets, or for the production of polycrystalline solar-cell base material. Since these products are prepared from hyperpure, molten silicon, it is necessary to melt solid silicon in crucibles. In order to make this process as efficient as possible, large-volume, solid pieces of silicon such as these polycrystalline rods have to be reduced in size prior to melting. Conventionally, this is always associated with superficial contamination of the semiconductor material, because the size reduction is carried out with metallic crushing tools, such as jaw crushers, crushing mills, hammers or chisels.
Efforts are being made to reduce the size of semiconductor rods without contamination. Prior art attempts to do this are found in German published patent application No. 2,262,178 and German published application No. 3,811,091, and in German Patent No. 3,428,255, according to which thermal stresses are generated in the semiconductor rod. This is achieved, for example, by heating the rod to temperatures from 600.degree. C. to 1,000.degree. C. and subsequent cooling rapidly in water, or by heating parts of rods in a microwave oven. Such thermally generated stresses can result in direct size reduction of the semiconductor rod. These stresses can also weaken the rod to such an extent that further size reduction is then possible with less-contaminating crushing tools which are made of, for example, plastic or silicon.
High temperatures initiate and/or accelerate diffusion processes, which can be a detriment of the purity of the semiconductor material. This is because at least some of the superficial impurities, especially foreign metals, reach the interior of the rod or the fragments and elude cleaning processes at the surface. Moreover, thermal size reduction methods may require further use of mechanical size reduction tools if there was insufficient size reduction due to thermally generated stress. Further, thermal treatment is laborious and expensive when it is required to provide size reduction of fragments which still exceed a specified maximum limit size.